Instead of city maps and fliers, Peapod's covered subway billboards with a 2-D version of grocery shelves. On them, shoppers will find a sampling of the site's 12,000-product inventory with everyday goods like soda, pasta and hygeine products.

The system works by using QR codes, which users can scan from their phones and add to their shopping carts on Peapod's mobile app.

Once they've selected their items, they can schedule home delivery from their phones or organize their shopping list for later.

This is Peapod's first metro launch since rolling out its pilot program in 15 Philadelphia train stations earlier this year. The site, which functions much like FreshDirect.com, is available in 12 states throughout New England and the Midwest.

First-time users in Chicago can save $20 on their first order using promo code "CHICAGORAIL."